Ethylene is a ubiquitous building block in the manufacture of a wide variety of chemical and plastic products. Ethylene is typically produced industrially by pyrolysis of hydrocarbons in a furnace in the presence of steam. The furnace effluent stream comprising a range of components is typically cleaned up, dried to remove water, compressed and passed to an olefins recovery section to condense the ethylene and other condensable heavy end components (ethane, propylene, propane, etc.). The condensed stream is then distilled to remove the light ends (methane and hydrogen) and fractionated to separate ethylene from the heavy ends.
Compositional range of the furnace effluent stream depends on several factors including the type of hydrocarbon feedstock used. A representative composition of the effluent of a furnace employing three different hydrocarbon feedstocks and operated to maximize ethylene formation is given in Table 1.
TABLE 1 ______________________________________ Effluent Composition (mole %) Furnace Feedstock Component Ethane Propane Naphtha ______________________________________ H.sub.2 35.9 20.5 15.8 CH.sub.4 6.5 27.8 26.5 C.sub.2 H.sub.4 34.3 32.0 33.6 C.sub.2 H.sub.6.sup.+ 23.3 19.7 24.1 ______________________________________
As can be seen, hydrogen and methane light end components comprise a substantial portion of the effluent. These light ends have an undesirable impact on the stream dew point temperature. Greater refrigeration power is required to condense out ethylene and other components from streams containing high hydrogen and methane concentration, and refrigeration makes up a significant portion of the process energy requirements. Additionally, in existing plants ethylene refrigeration availability may be limited and therefore a process bottleneck to any increase in ethylene output.
It would be desirable to compensate for the presence of light end components to obtain greater condensation against propylene refrigeration. As far as applicant is aware, in an ethylene plant employing hydrocarbon pyrolysis, it has been heretofore unknown to reinject liquid hydrocarbons, particularly C.sub.2 -lean liquid hydrocarbons from the liquid driers, deethanizer and/or depropanizer into the reactor effluent stream for the purpose of raising the stream dew point temperature, lowering refrigeration energy usage and shifting cooling requirements from ethylene refrigeration to propylene refrigeration.